1. Field of the Invention
The present invention relates to an air-fuel ratio control apparatus for an engine for controlling a fuel injection amount so that an air-fuel ratio of a mixture gas which is supplied to the engine is set to a stoichiometric air-fuel ratio.
2. Description of the Related Background Art
Hitherto, there has been disclosed an air-fuel ratio control apparatus for an engine in which a first oxygen concentration sensor (hereinafter referred to an air-fuel ratio sensor) which can obtain a detection signal which is linear to an air-fuel ratio of a mixture gas which is supplied to the engine is provided on the upstream side of a 3-component catalytic converter arranged in an exhaust pipe and a fuel injection amount is controlled so that an air-fuel ratio is set to a stoichiometric air-fuel ratio in accordance with the detection signal from the air-fuel ratio sensor, wherein a second oxygen concentration sensor (referred to an O.sub.2 sensor) which can obtain a rich/lean detection signal for the air-fuel ratio of the mixture gas which is supplied to the engine is provided side by side with the air-fuel ratio sensor on the upstream side of the 3-component catalytic converter, and a deviation between an actual air-fuel ratio and the detection signal of the air-fuel ratio sensor is corrected on the basis of the detection signal from the O.sub.2 sensor (for instance, refer to JP-A-56-64l25).
However, in the case where the O.sub.2 sensor is provided on the upstream side of the 3-component catalytic converter and a deviation between the actual air-fuel ratio and the detection signal of the air-fuel ratio sensor is corrected by the detection signal of the O.sub.2 sensor as in the above conventional apparatus, there are the following problems.
1 To raise a purification factor of the 3-component catalytic converter, the air-fuel ratio is controlled in a manner such that the rich and lean air-fuel ratios are repeated at a short period with respect to the stoichiometric air-fuel ratio as a center value. In the case where the O.sub.2 sensor is provided on the upstream side of the 3-component catalytic converter, the detection signal of the O.sub.2 sensor changes so that the rich (R) and lean (L) values are repeated at a short period as shown in (a) in FIG. 3. Therefore, if the air-fuel ratio is corrected on the basis of the detection signal of such a short period, since the air-fuel ratio is influenced by a fluctuation of the detection signal, the air-fuel ratio cannot be stably controlled.
2 In the upstream of the 3-component catalytic converter, the exhaust gas is not sufficiently mixed. Therefore, the detection signal of the O.sub.2 sensor is easily influenced by a certain special cylinder in dependence on the attaching position or the like.
3 A temperature is high in the upstream of the 3-component catalytic converter. A copper component is included in the exhaust gas. Therefore, the O.sub.2 sensor itself for correction remarkably deteriorates.